1. Field of the Invention
The present invention relates to a method and apparatus for measuring position by image processing whereby the relative position of an object whereon a light-emitting element is arranged is measured by detecting the position of the light-emitting element on an image and using the detected position of the light-emitting element.
2. Description of the Related Art
When loading a load from an unmanned loader onto an unmanned dump truck at an outdoors work site, it is necessary to ascertain the precise position of the unmanned dump truck. For this purpose, the image of a specific portion of the dump truck (object) is picked up by means of a camera mounted at the unmanned loader, and the results of this image pick-up process are used to determine the precise position of the dump-truck.
In a well known example of such technique, a light-emitting element is mounted on a specific portion of an object whose position is to be measured in order to facilitate identification of the position of the object; the image of the object and around it is picked up by a camera; the picked-up image is processed to convert it into light/dark binary form; position of high lightness is found as the position of the object (light-emitting element) on the screen; and the relative position of the object with respect to the camera is thereby measured.
The above-mentioned method that picks up the image of the background around the object together with the light-emitting element can be adopted with no problem indoors or in an outdoor site whose lightness environment is stable. However, in an outdoor environment where there are large fluctuations of ambient lightness, precise detection of the position of the light-emitting element in the picked-up image i.e. the position of the object has sometimes proved impossible. That is, even though the threshold level for identifying locations of high lightness on the image is set while considering the background lightness, fluctuation of the background lightness may cause high lightness portions of the background to be erroneously detected as the location of the light-emitting element.
In order to avoid the effect of fluctuation of the background lightness, Japanese Patent Publication (Kokai) No. 6-176143 discloses a technique of measuring the position of an object in which an image of the object is picked up by a camera in two consecutive frames each for a prescribed time and a difference image is acquired from these two frames. Then, the difference image is processed to convert it into binary form and the position of the object is measured based on the binirized difference image.
However, this published technique does not disclose that a light-emitting element is provided on the object. Further, the purpose of the published technique is to detect the movement of a moving object. With this published technique also, just as with the technique described above, noise is likely to generate in the processing system due to fluctuations of ambient lightness, and therefore, stable processing results could not be obtained.